A long and flexible conduit made of a synthetic resin is typically connected to an exhaust port of a water heater in order to guide high-temperature gas discharged from the exhaust port of the water heater (arranged inside a building) to the outdoors. Conventional corrugated conduits made of a synthetic resin are formed in a bellows shape having concave portions and convex portions alternately and continuously provided in the length direction, and are freely bendable. Such conduits made of a synthetic resin are formed, for example, through a vacuum molding or blow molding process, and have flexibility and lightweight properties.
An exemplary hot water supply system is disclosed in US2016/60873A1. In particular, one end of a conduit is connected to an exhaust port of a water heater. The conduit extends indoors and is laid in a bent state. A building is provided with an air diffusion part (diffuser) penetrating its outer wall or roof and communicating with the exterior. The other end of the conduit is connected to this air diffusion part. The conduit laid so as to be bent indoors carries an exhaust gas from the water heater, guides the exhaust gas to the air diffusion part and releases the exhaust gas via the air diffusion part to the outside air. An outer tube for protecting the conduit is laid between the air diffusion part and the water heater so as to insert the conduit therein. In general, the outer tube is composed of a plurality of straight metallic cylinders including an aluminum alloy, steel or the like. The plurality of straight metallic cylinders are joined by means of a screw so that the outer tube is bent along the route of the conduit. The screw penetrates the metallic cylinders so that the head of the screw is positioned on the outer surface side of the metallic cylinders. As a result, a sharp tip end of the screw protrudes inward from the inner surfaces of the metallic cylinders.
The hot water supply system is constructed in such a manner that an outer tube, which is bent, is laid between the water heater and the diffuser and that the conduit is inserted into the outer tube. The conduit is introduced inside through an opening at one end of the outer tube. When the conduit passes through the inside of the outer tube, the tip end of the screw protruding on the inner surface of the outer tube can abut against the outer surface of the conduit. The outer surface of the conduit is easily damaged due to such contact between the outer surface of the conduit and the screw. Especially, the edges of the convex portions of the conduit are damaged by the sharp tip end of the screw so that a hole is easily formed in the conduit. The presence of the hole causes disadvantageous leakage of a gas from the conduit to the outside, or when a hole is found in the conduit, the conduit must be replaced with a new one. In order to solve the above problem, the present invention provides a conduit having high resistance to scratching by a screw.